1. Field of the Invention
The present invention relates generally to DC/DC converters and more particularly to charge pumps.
2. Description of the Related Art
Charge pumps are DC/DC converters which are characterized by a pump capacitor and a reservoir capacitor embedded in a switch network. The reservoir capacitor provides an external load with voltage and current. The switch network is typically configured to conduct current between a supply voltage and the pump capacitor and, subsequently, to conduct charge between the pump capacitor and the reservoir capacitor.
Charge pumps are particularly suited for use in integrated circuits where space and supply voltages are limited. For example, complimentary metal oxide silicon (CMOS) circuits typically require positive and negative voltage sources (V.sub.DD and V.sub.SS) but space limitations generally prohibit the availability of more than one voltage supply. Because this available input voltage often has a small amplitude V.sub.in, a charge pump which can generate tripled and inverted output voltages, i.e., 3V.sub.in and -3V.sub.in, is particularly preferred. Such a charge pump also preferably operates with a simple, efficient two-phase switching operation and has a small parts count.
Although DC/DC converter configurations are available for generating doubled voltages (e.g, U.S. Pat. No. 5,029,063), inverted voltages (e.g., U.S. Pat. No. 4,982,317), doubled and inverted voltages (e.g, U.S. Pat. No. 4,807,104), multiplied and inverted voltages (e.g, U.S. Pat. No. 5,306,954) and tripled voltages (e.g., U.S. Pat. Nos. 4803,612, 5,095,223 and 5,111,375), they generally require more than two switching phases and/or a large number of parts.